Command Line Basics

When we speak of the command line, we are working in what is known as a shell. A shell is a program that passes typed commands to the computer’s operating system to execute. The following is a list of popular shells that you will encounter:

Current shell - echo $0

Default shell - echo $SHELL

There are many different *nix versions and cousins. The flavors include *BSD, Linux, Unix, and others. Each version and implementation has some unique features to their shells. Some popular versions and distributions are:

yum install <package>

apt-get install <package>

(yum is in the process of being replaced by dnf)

This is not navigation, but you need to know this

Commands are often followed by one or more switches at the command line. When one - is used, each character following it is an option. When two '--' are used, the full string is considered the option.

ls -help attempts to run the ls command with the h,l,p options*

ls --help prints the ls help file to the screen

(*e is invalid, but needed for the demo)

The following is the output of ls -lah (long list,all files,human readable)

(Diagram from linuxcommand.org)

rwx

chmod changes the permissions of the file or directory

vim filename - opens the designated file in normal mode

:help - displays the vim help file

/texttosearch - search for text in the document (case sensitive)

i - insert mode to edit the file

esc - return to command mode

:w - write/save the file to disk

:q - quit the editor if no changes were made or unsaved

:wq - write/save the file and quit the editor

:q! - quit the editor without saving the changes

cp filename /path/to/copy/to

cp filename newfilename

copies the file from the current location/name to the new location/name

cp -R /path/to/directory /path/to/new/directory

copies the the files and directories from the specified directrory to the new location

mv filename /path/to/new/location

mv filename newfilename

moves the file from the current location/name to the new location/name.

mkdir directoryname

mkdir -p path/to/new/direcctory

mkdir -v

mkdir -m=744 directory

creates directories to store files and directories

rm filename

removes (deletes) the indicated file

rm -rf directoryname

removes (deletes) the indicated directory and all of its contents, including hidden files

find . -name 'filename.txt'

Searches for the filename.txt file in the current directory and sub directories

Change . in the above command to a path to start at that path and search its sub directories

Other popular factors to search on include owner, time, type, size, file types

grep -iR pattern Documents/

Searches files for the phrase pattern in a case insensitive(i) manner in and below(R) the local Documents directory.

head -n 15 filename.txt

head displays the first lines of a file (10 by default, 15 in the above example)

tail -n 15 filename.txt

tail displays the last lines of a file (10 by default, 15 in the above example)

tail -f filename.txt

The above use of the tail command with the -f flag continually rereads and displays the end of the file. This is useful when monitoring a log file in real time, for example. Use <ctrl>-c to stop reading the file.

The service and systemctl commands control the status of services on the system. The following examples are for controlling the Apache web server status.

service httpd {start,stop,restart,status}

systemctl {start,stop,restart,status} httpd

less filename

Displays the contents of a file in the terminal window.

Use the up and down arrows to navigate the file.

Use the v key to edit the file in the system’s default editor. When you quit the editor, you return to the less process.

Use a forward slash followed by text to search for to find particular types.

/texttosearchfor

tar zxvf filename.tar.gz

Extracts the contents of the tar.gz file to the current directory

tar zcvf newcompressedfile.tar.gz file1 file2 file3

Creates a new compressed file containing all the files indicated in the command

tar zcvf newcompressedfile.tar.gz /path/to/files

Creates a new compressed file containing all the files in the indicated directory

sudo ls /var/log/messages

By default, runs a command as the root user. Adding -u <user> will specify a different user to run the command as.

The pipe | symbol passes the output of a command to another command.

The following command will list the contents of the current directory in a long format. The grep command then filters and displays only the lines of text that contain the word filename in a case insensitive manner.

ls -l|grep -i filename

The following command will run the ps -ef command and show the results that match the phrase ssh on the screen

ps -ef|grep ssh

man <command>

The man command displays the manual (help) page for the command indicated.

Perhaps the most useful and under-utilized command in this presentation.

ssh someuser@example.com

ssh someuser@webdevbox

ssh someuser@192.168.1.1

ssh myotherbox

The above commands open a connection from the current computer to a remote computer. You will be prompted for a password or utilize a matching ssh key for authentication. Once connected, all commands you execute are performed on the remote machine. If a username is specified, you are connected as that user. If no username is specified, your current username is assumed.

ssh someuser@example.com systemctl restart httpd

The above command will log into the remote machine as someuser and run the command specified. In this case, it is restarting the Apache webserver.

These are commands and concepts that are useful to know but may fall into niche categories. Many of these will likely be useful at some point to this audience.

These commands should not be used unless you really understand what you’re doing with them.

rm -rf / - Will delete the entire file system

mv file /dev/null - will move the file to a system device that will delete the file

:(){:|:&};: - a fork bomb which creates a function and executes it until the system freezes

$COMMAND > /dev/sda - overwrites data on the block device, in this case the main drive

mkfs.ext4 /dev/sda - formats the block device using the ext4 filesystem, in this case the main drive. mkfs.* may be a variety of filesystem types.

Bash is probably the most commonly used shell on *nix systems. Similar files will exist for other shells.

.bashrc - configuration file for non-login shells

.bash_profile - configuration for login shells

.bash_history - history of the recent commands run in the bash shell

log files are your friend

Common Location — /var/log

Read recent system messages — tail /var/log/messages

Print Kernel messages — dmesg

Links to this presentation